Earlier studies were directed to developing sacrificial acceptor-donor systems for producing hydrogen and/or oxygen from water via visible light irradiation. Redox catalysts were developed which have the effect of mediating water reduction and oxidation.
Later, two types of catalysts were combined to render unnecessary the utilization of sacrificial acceptors and donors. The advantage of this system is its simplicity. Apart from the sensitizer and the two catalysts, the system contains only one other component, namely, an electron relay substance.
However, the simultaneous production of gaseous products such as hydrogen and oxygen gives rise to the problem of separating one from the other in a practical and convenient manner.